1. Field of the Invention
The present invention relates to the field of Insensitive Munitions (IM), and, more particularly, to a new and simplified mechanism for reducing the vulnerability of propellant loaded cartridges from suffering explosive events due to unplanned thermal stimuli.
2. Description of Related Art
Newer munitions are designed to minimize a violent, i.e. explosive, response when subjected to unintended stimuli during transportation and storage, such munitions are generally known as insensitive munitions or simply as, IM. Generally, such IM type munitions are provided with means to vent themselves—such that when unintentionally subjected to heat or kinetic energy from an accident, a fire, or as the result of enemy action—the venting avoids pressure build-up if the cartridge propellant is ignited and the propellant will then tend to burn rather than explode.
Standards have been established for Insensitive Munitions, wherein the particular munition must pass Fast Cook-Off (FCO) and Slow Cook-Off (SCO) test requirements of MIL-STD-2105B, “Military Standard for Hazard Assessment Tests for Non-Nuclear Munitions”. In a typical Fast Cook-Off test, the munition is engulfed in the flames of a jet fuel (or gasoline) fire exhibiting a minimum average temperature of 1,600.degree. F., to assess its response to rapid heating. In the Slow Cook-Off test, the munition is heated in a closed chamber at a linear rate of 6.degree. F. per hour until a reaction occurs, to assess its response to gradual heating. The FCO and SCO tests are considered to be passed if the munition exhibits a Type V response where the test items only burn or scatter lethal fragments no further than 50 feet from the burn pan or test oven.
The concept of venting to avoid pressure build-up and eliminate the danger of igniting surrounding material or atmosphere was disclosed in U.S. Pat. No. 2,644,059, issued Jun. 30, 1953, to Jones, with respect to electric fuses enclosed within a cartridge. In this case, if the fuze link were blown by an over-current or even by a short circuit, the gas pressure created thereby would be vented through sections of the ends of the cartridge which included a porous type metal known as Porex—a metal capable of withstanding the pressures that are built up within the cartridge while allowing the pressures to be bled off.
While the solution of U.S. Pat. No. 2,644,059 of using a porous type metal sufficed to vent the relatively small pressure build-up within a fuze, it is not adequate to provide the venting for an event impacting the propellant within a cartridge itself, or the propellant within rocket, or an explosive loaded cartridge—where significant venting is required to avoid pressure build-up and an explosive situation. In contrast, U.S. Pat. No. 7,025,000 (Wong et al.) does provide such relatively massive venting —disclosing use of a plastic threaded adaptor having a melting temperature that is lower than the auto-ignition temperature of the explosive within the particular munition. This adaptor secures a fuze or metal closing plug to an explosive loaded projectile and is designed to permit significant venting of combustion gases through the nose of the projectile upon auto-ignition of the explosive, thereby providing an enlarged venting area which in turn prevents detonation of the explosive and fragmentation of the projectile body. Similar to U.S. Pat. No. 7,025,000—U.S. Pat. No. 6,338,242 (Kim et al.) discloses a thermoplastic warhead adaptor which melts releasing a dome plug to provide again a significant vent opening, to reduce the danger of explosion from heat induced over-pressurization in rocket motors.
Other, similar patents, disclose munition venting systems to avoid explosive catastrophes. U.S. Pat. No. 3,927,791 (Hershberger) discloses a fusible plug assembly of a bismuth alloy that melts at approximately 210.degree. F. to permit venting. U.S. Pat. No. 4,557,198 (Hickey) discloses a venting aperture with two retaining means and a shear pin configuration. U.S. Pat. No. 4,991,513 (Malamas, et al.) discloses a plurality of vent holes in the nose section that are open when coincident with vent holes in a collar. In storage, the vent holes are left open with the placement of a safety pin, which is removed immediately prior to loading. U.S. Pat. No. 5,035,180 (Purcell, et al. '180) discloses a venting system having a metal patch attached to the casing that shears from the casing when heated. U.S. Pat. No. 5,035,182 (Purcell, et al. '182) discloses a vent system having a bi-metallic patch attached to the casing that deforms with heating, which then separates from the casing. U.S. Pat. No. 5,155,298 (Koontz) discloses a solder plug that forms the primary load carrying portion of a release mechanism mounted on the external side of a closure plate whose internal side holds the explosive within the warhead. Upon an unforeseen thermal event, the plug melts, no longer maintaining an expandable snap ring in an expanded position, such that the expanded snap ring retracts—without the closure force of the snap ring, a set of radial set screws will shear away, freeing an adapter plate that confines the closure plate against the explosive, thereby venting the pressure within the warhead case. U.S. Pat. No. 5,311,820 (Ellingsen) discloses a melting fusible material that allows a free-loaded spring retainer to push the melted or liquid fusible material out of a set screw hole, causing the nozzle to separate from the case at the interface. U.S. Pat. No. 5,337,672 (Boissiere, et al.) discloses a set of locking screws that secures a casing to a plug, with the locking screws designed to shear at a given pressure. U.S. Pat. No. 5,398,498 (Mort, et al.) discloses a fusible helical joint member made of a metallic material having a low melting point that melts when heated, allowing an adapter ring to disconnect the warhead from the rocket motor. U.S. Pat. No. 5,735,114 (Ellingsen) discloses a bimetallic retaining ring that releases the engagement between two or more sections of a rocket motor when contacted by an external heat source.
The various venting system solutions detailed above are uniformly not adequate for providing insensitive munition venting for centerfire medium (e.g. 25 mm, 30 mm, 50 caliber), and even large (e.g. 105 mm) caliber cartridges—wherein: (1) there is a physical environment, within a gun barrel, that does not allow for any mechanical devices about the cartridge; (2) the venting system must withstand pressures from about 72,000 up to about 90,000 psi without failing when the cartridge is fired and still provide adequate area for quick venting, i.e. simple plastic or metal closures, or patches, or the like, are not adequate; (3) due to the vast numbers of cartridges produced, the solution must be inexpensive, and (4) the solution cannot significantly change the configuration or weight of presently mass produced cartridges.